When a solid-state crystal is exposed to ionizing radiation, several absorption bands make their appearance at increasingly higher levels of radiation damage. In the case of the alkali halides, the F-band is the first of the radiation damage centers to produce a detectable absorption band. With increasing radiation exposure, a second damage center builds up in the absorption spectrum. The second damage center is known as the M-center and is generally thought to consist of two adjacent F-centers. Absorption measurements in LiF indicate that the peak of the M-center absorption occurs at 443 nm.
Highly purified lithium fluoride (LiF) crystals have long been used as optical windows. Optical grade LiF is known for its excellent transmission from the deep ultraviolet through the infrared. Lif crystals have been used for high-level (Mega-Rad) gamma dosimetry using radiation-induced absorption peaks measured with a spectro-photometer. However, absorption measurements are a very insensitive way to measure these radiation damage centers.
Alkali halides have been used to measure the dose of radiation to which crystalline material has been exposed by exciting the crystalline material with optical radiation at a wavelength that corresponds to that of a known absorption band and observing the luminescence.
Although the F-center provides the greatest concentration of damage centers for a given radiation exposure, the luminescence of the F-center is thermally quenched below room temperature and therefore it is of limited usefulness in quantifying ionizing radiation exposure using luminescence techniques. However, excitation into the longer wavelength M-center absorption band at room temperature produces luminescence. M-center luminescence involves a substantial Stokes' shift, allowing the luminescence to be observed at a significantly different wavelength from the exciting wavelength.
D. F. Regulla, Lithium Fluoride Dosimetry Based on Radiophotoluminescence, Health Physics, Vol. 22, pgs. 491-496 (1972) discloses a dosimetry technique in which luminescence of doped LiF that has been irradiated is excited by light at 450 nm. The luminescence spectrum includes peaks at 520 nm and 620 nm.
S. Datta and A. E. Hughes, Luminescence Dosimetry Using Sodium Fluoride Single Crystals, Health Physics, Vol. 29, pgs. 420-421 (1975) discloses an investigation using NaF excited by light at 350 nm and with a luminescence peak at 660 nm.
In the case of the work reported by Regulla, the LiF that is irradiated is doped. The peak that is reported at 520 nm is not observed with pure LiF.
The co-pending application discloses that radiation damage can be quantified by measuring the M-center luminescence. The peak of the M-center emission spectrum in LiF occurs at about 665 nm. In accordance with the co-pending application, excitation of an LiF crystal with light at a wavelength that closely matches the wavelength at which the peak of the M-center absorption occurs was found to significantly amplify the luminescence yield of the M-center of LiF.
A disadvantage of the techniques mentioned above is that they require excitation sources that emit light at relatively short wavelengths. Lasers that emit light at such short wavelengths are much more expensive than lasers that emit light in the red and infrared regions.
The applicant is not aware of any published work in which the peak of the M-center absorption band of sodium fluoride (NaF) is reported.